1. Field of the Invention
The present invention relates to ultrasonic imaging. More specifically, the present invention relates to a median temporal filter which is used in connection with ultrasonic imaging, especially, for applications such as angiography.
2. Background Information
Various applications of ultrasonic imaging within living subjects have included the mapping of bloodflow within a subject under examination. Some applications for this technology include angiography studies or the mapping of blood vessels. Angiography has been a recent development in ultrasonic imaging, especially in systems such as the Spectra.TM. VST Masters Series brand ultrasonic imaging system available from Diasonics Ultrasound, Inc., of Milpitas, California which includes the UltrasoundAngio.TM. brand angiography modality. Such systems typically employ a front-end gain to the Doppler signal and then assign a color index to the power level of the Doppler signal received. This new color index signal is then passed to a temporal filter which averages the signal in order to eliminate any pulsatile information and increase the Signal to Noise Ratio (SNR). The implementation of such systems, especially for angiography studies, has encountered some problems with artifacts.
One main source of artifacts in angiography systems, as described above, are tissue flash and probe motion. Because probe motion is normally sustained for a large number of frames, it can usually be repressed only by blanking Doppler signals with frequency shifts below a certain threshold. When the probe becomes stationary, however, the sonographer can then study a fixed region and the problem normally resolves itself.
Tissue flash, however, is the result of either respiration of the patient or the systolic phase of the cardiac cycle. Since there is no way to repress these events, tissue flash provides a significant artifact in such a system.
FIGS. 1 and 2 illustrate an example of tissue flash as it occurs in a subject under examination in a ultrasonic imaging system during angiography studies. FIG. 1 illustrates a time versus displacement motion of the posterior wall of the carotid artery during the systolic phase of the patient's heartbeat. Note that the wall moves out quickly and retracts relatively slowly. As a result, the average frequency shift of the received Doppler signal from the vessel wall remains relatively high for a short period of time. Because the high pass filter typically applied to the Doppler signal exhibits a large gain gradient near the stopband, the power at the output of the filter produces a series of impulse-like outputs at the cardiac rate, as shown in FIG. 2.
As already discussed, one prior art method of reducing the effect tissue flash has upon the resulting signal is by using a linear filter. Linear filtering is used in the UltrasoundAngio system in the form of a frame-to-frame single-pole filter. This filter reduces the noise in the image by the following relationship: EQU SNR Gain dB=10 log (1/(1-.alpha.))
where .alpha. is the pole location of the filter wherein .alpha. is set to a value to approximate the cardiac cycle of the subject (e.g., typically wherein approximately 75%-90% of the previously displayed signal is used to generate the new signal). Unfortunately, when a linear filter encounters tissue flash impulses, it distributes them temporally, effectively retaining them on the screen for long periods of time. The amount of suppression achieved by the filter is minimal if the impulse amplitude is much larger than the desired blood flow signals. Obviously, the linear filter is not ideal for dealing with tissue artifacts.
Thus, although such linear filters typically have unique advantages, especially in the application of typical prior art Doppler color flow imaging for angiography, such linear filters also possess inherent disadvantages. Prior art Doppler color flow imaging typically maintains the pulsatility of data. Angiography has no such requirement. Thus, prior art techniques of linear filtering, especially for Doppler color imaging in a living subject, possess certain disadvantages which are neither suitable for nor desirable for performing angiography by way of ultrasonic imaging.